Method to direct differentiation of pluripotent stem cells into functional heart muscle

ABSTRACT

The present invention is directed to a method for producing bioengineered heart muscle (BHM) from pluripotent stem cells, generally comprising the steps of inducing mesoderm differentiation, cardiac differentiation, and cardiac maturation by directed tissue formation. The method is a robust, serum-free and reproducible way to produce BHM for multiple applications, and closed herein, as well as to uses of said BHM in pharmacologic and toxicity screenings, and its use in medicine.

BACKGROUND OF THE INVENTION

Human pluripotent stem cells (hPSCs) are now widely used to provide a theoretically endless and also large supply of human cardiomyocytes (Kehat et al. J Clin Invest 108, 407-414 (2001); Takahashi et al. Cell 131, 861-872 (2007); Zhang et al., Circ Res 104, e30-41 (2009)). Human cardiomyocytes have been derived from human embryonic stem cells (hESCs) (Thomson et al. Science 282, 1145-1147 (1998)) and induced pluripotent stem cells (hIPSCs) (Takahashi et al., Cell 131, 861-872 (2007)) and have a demonstrated use for multiple purposes including developmental models (Lian et al. Stem Cells 2012 (2012)), drug efficacy and/or safety screening (Schaaf et al. PLoS ONE 6, 20 (2011)), hypertrophy modelling and regenerative applications. Additionally, with recent advances in hIPSC technology, cardiomyocytes exhibiting heritable genetic disease phenotypes can be generated in vitro (Carvajal-Vergara, X. et al. Nature 465, 808-812 (2010); Itzhaki et al., Nature 471, 225-229 (2011); Malan et al. Circ Res (2011); Moretti et al., N Engl J Med 363, 1397-1409 (2010); Yazawa et al. Nature 471, 230-234 (2010)).

It is now widely accepted that the low density 2D culture of biopsy-derived human cardiomyocytes leads to rapid changes in cardiomyocyte phenotype and morphology (Bird et al. Cardiovasc Res 58, 423-434 (2003)) and making it difficult to extrapolate results to the in vivo situation. In order to obtain a cardiomyocyte phenotype more representative of in vivo conditions, cardiac tissue engineering has been used (Eschenhagen et al. FASEB J 11, 683-694 (1997); Zimmermann et al. Biotechnol Bioeng 68, 106-114 (2000), Zimmermann et al. Circ Res 90, 223-230 (2002); Tulloch et al. Circ Res 109, 47-59 (2011); Tiburcy et al. Circ Res 109, 1105-1114 (2011); Eschenhagen et al. Am J Physiol Heart Circ Physiol 303, 11 (2012)) to generate constructs with similar properties to the native heart tissue.

The current ideology of tissue engineering is to generate/isolate the required cell type(s), and seed them into an engineered environment to promote their differentiation and generate in vivo-like tissues. Tissue engineering may therefore be considered as an inefficient process for two reasons, 1) disassociation of a tissue/differentiation culture destroys the extracellular environment thus destroying developmental information (eg. cell-cell interconnectivity, geometric cell positioning, cell-ECM connectivity), this necessitates very large increases in extracellular matrix (ECM) production in order to re-build the environment (Hudson et al. Tissue Eng Part A 17, 2279-2289 (2011)), and 2) the disassociation process is variable between hPSC lines and can lead to considerable cell death.

Other protocols reported in the literature may require modification of the protocol to enable similar cardiomyocyte efficiencies in multiple hPSC lines. However, the inventor's results demonstrate that changes in differentiation protocol may greatly affect the cardiomyocyte phenotype (e.g. it is shown that dorsomorphin may greatly affect the bioengineered heart muscle (BHM)). This may lead to changes in tissue engineered myocardial properties which may mask the effects of different experimental conditions or genetic disease models, therefore care must be taken when using different protocols in different lines.

Some recently published protocols may enable the same protocol to be used for multiple lines, they also produce cardiomyocytes with very high purity. However, pure cardiomyocytes do not facilitate the formation of functional tissue engineered myocardium and both cardiomyocytes and stromal cells are required for the formation of functional tissue engineered myocardium (Naito et al. Circulation 114, 172-78 (2006), Hudson et al. Tissue Eng Part A 17, 2279-2289 (2011)).

Hence, there is a need in the art for methods for producing bioengineered human myocardium, which are capable of overcoming the above disadvantages.

The development of a robust differentiation protocol is a very important step allowing the consistent production of BHM. In this study n>140 BHM in >18 independent experiments were produced and every one exhibited spontaneous beating activity. Additionally, the protocol enables to produce BHM from multiple hPSC lines using the same protocol. In addition, all disassociation steps could be eliminated and hPSCs were differentiated directly into bioengineered myocardium, thus retaining the developmental memory of the tissue, prevent any tissue recreation response and provide a more accurate in vitro model of human myocardial development.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing bioengineered heart muscle from pluripotent stem cells, comprising the steps of

-   -   (i) cultivating pluripotent stem cells in a basal medium         comprising an effective amount of (a) BMP4, Activin A, FGF2, a         GSK3-inhibitor, and (b) a serum-free supplement resulting in a         final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml         transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium         selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml         Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1         μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm         differentiation of said pluripotent stem cells;     -   (ii) cultivating the cells obtained in step (i) in a basal         medium comprising an effective amount of an inhibitor of the         Wnt-signaling pathway and a serum-free supplement as defined in         step (i), thereby inducing cardiac differentiation of the cells;         and     -   (iii) cultivating the cells obtained in step (ii) in a basal         medium comprising an effective amount of a serum-free supplement         as defined in step (i), under mechanical stimulation, thereby         promoting cardiac maturation.

Carrying out the method disclosed herein human pluripotent stem cell (hPSC) derived bioengineered heart muscle (BHM) is generated by directed tissue formation of hPSCs in collagen hydrogels. To form BHM, in vivo development was mimicked using a directed serum-free induction protocol causing the tissue to progress through distinct, known developmental stages, through pluripotency, early mesoderm, cardiac progenitor, immature cardiomyocytes and finally to more mature cardiac tissue comprised of 50% cardiomyocytes, with the rest being predominately a stromal cell fraction. The inventors optimized their serum-free BHM protocol and found that individual BHM properties are highly dependent on particular stimuli, thus indicating that multiple exogenous stimuli are required for optimal BHM properties. In the end rhythmically contractile BHM was produced with measurable contractile force, ability for pacing and inotropy in response to increased resting length, calcium concentration and β-adrenergic stimulation. This BHM protocol, without modification, was capable of consistently producing BHM from multiple hPSC lines (in every BHM in every experiment conducted).

The present data suggests that the BHM protocol disclosed herein is a robust, serum-free and reproducible way to produce human myocardium for multiple applications.

For example, it is also demonstrated that BHM is a potential model of human myocardium development, and shown that inhibition of BMP signalling leads to a more immature cardiac phenotype with reduced contractile strength.

Accordingly, the present invention is also directed to a BHM produced by the method according to the invention.

Further contemplated is the use of the BHM according to the invention in an in vitro-model for drug toxicity screening. In other words, the present invention is also directed to a method for screening drug toxicity, comprising the step of contacting a BHM according to the invention with a drug to be screened.

Moreover, the present invention is directed to the use of the BHM according to the invention in an in vitro method for testing of cardiac function modulation by pharmacological candidate agents. Thus, also described is a method for testing of cardiac function modulation, comprising the step of contacting a BHM according to the invention with a pharmacological candidate agent.

Finally, the present invention is also directed to the use of the BHM according to the invention as a research tool, as well as to a BHM according to the invention for use in medicine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for producing bioengineered heart muscle from pluripotent stem cells, comprising the steps of

-   -   (i) cultivating pluripotent stem cells in a basal medium         comprising an effective amount of (a) BMP4, Activin A, FGF2, a         GSK3-inhibitor, and (b) a serum-free supplement resulting in a         final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml         transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium         selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml         Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1         μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm         differentiation of said pluripotent stem cells;     -   (ii) cultivating the cells obtained in step (i) in a basal         medium comprising an effective amount of an inhibitor of the         Wnt-signaling pathway and a serum-free supplement as defined in         step (i), thereby inducing cardiac differentiation of the cells;         and     -   (iii) cultivating the cells obtained in step (ii) in a basal         medium comprising an effective amount of a serum-free supplement         as in (i), under mechanical stimulation, thereby promoting         cardiac maturation.

In a preferred embodiment, the pluripotent stem cells are pluripotent stem cells of primate origin, more preferably the pluripotent stem cells are human pluripotent stem cells. Pluripotent stem cells are able to differentiate into every cell type of the body. As such, human pluripotent stem cells offer the unique opportunity to obtain bona fide human heart cells. Currently, the most utilized pluripotent cells are embryonic stem cells (ESC) or induced pluripotent stem cells (iPSC). Human ESC-lines were first established by Thomson and coworkers (Thomson et al., Science 282: 1145-1147 (1998); incorporated herein in its entirety by reference). Human ESC research recently enabled the development of a new technology to reprogram cells of the body into a ES-like cell. This technology was pioneered by Yamanaka and coworkers in 2006 (Takahashi & Yamanaka Cell 126: 663-676 (2006); incorporated herein in its entirety by reference). Resulting induced pluripotent cells (iPSC) show a very similar behavior as ESC and, importantly, are also able to differentiate into every cell of the body. Moreover, it was reported that also parthenogenetic stem cells are likely to be suitable for BHM-production (Didie et al. J Clin Invest. 123, 1285-1298 (2013); incorporated herein in its entirety by reference). Accordingly, the pluripotent stem cells can be selected from embryonic stem cells, induced pluripotent stem cells, and parthenogenetic stem cells. In the context of the present invention, said pluripotent stem cells are however not produced using a process which involves modifying the germ line genetic identity of human beings or which involves use of a human embryo for industrial or commercial purposes.

The basal medium used in step (i), can be selected from DMEM/F12, StemPro, Iscove's medium, aMEM, DMEM, and RPMI. Preferably, the basal medium used in step (i) is RPMI supplemented with pyruvate. However any suitable basal medium may be used in the method. Basal mediums are commercially available or may be prepared according to recipes which are publicly available, e.g. from catalogues of the ATCC. If deemed appropriate, the basal medium may be supplemented with non-essential amino acids. If aMEM is used as the basal medium, the basal medium need not be supplemented additionally with non-essential amino acids. The non-essential amino acids are commercially available as a combined supplement. Such a supplement for example comprises 750 mg/L glycine, 890 mg/L L-alanine, 1320 mg/L L-asparagine, 1330 mg/L L-aspartic acid, 1470 mg/L L-glutamic acid, 1150 mg/L L-proline, and 1050 mg/L L-serine.

As set out above, the basal medium of step (i) comprises an effective amount of BMP4, Activin A, FGF2, and a GSK3-inhibitor. For example, such a basal medium comprises 1-20 ng/ml BMP4, preferably 2-15 ng/ml, more preferably 2.5-10 ng/ml, more preferably 3-8 ng/ml, most preferably 4-6 ng/ml, and even most preferably about 5 ng/ml;

0.1-10 ng/ml FGF2, preferably 1-9 ng/ml, more preferably 2-8 ng/ml, even more preferably 3-7 ng/ml, most preferably 4-6 ng/ml, and even most preferably about 5 ng/ml;

1-20 ng/ml Activin A, preferably 2.5-18 ng/ml, more preferably 5-16 ng/ml, even more preferably 7.5-14 ng/ml, still more preferably 8-12 ng/ml, most preferably 8.5-10 ng/ml, and even most preferably about 9 ng/ml.

The GSK3-inhibitor in the basal medium of step (i) can be selected, for example, from the group consisting of CHIR99021, CHIR98014, SB216763, TWS119, Tideglusib, SB415286, and LY2090314. However, any GSK3-inhibitor suitable in the method of the invention can be applied. In a preferred embodiment, the GSK3-inhibitor in the basal medium of step (i) is CHIR99021.

It will be understood by the skilled person that the concentration of an effective amount of a GSK3-inhibitor varies with the availability and inhibition constant of the inhibitor in question. In the context of the present invention, the term “effective amount” as used herein in the context of a GSK3-inhibitor is intended to mean an enzyme inactivating concentration. For example, in case of CHIR99021, the basal medium in step (i) comprises 0.1-10 μM CHIR99021, preferably 0.2-9 μM, more preferably 0.3-8 μM, even more preferably 0.4-7 μM, still more preferably 0.5-6 μM, more preferably 0.6-5 μM, more preferably 0.7-4 μM, more preferably 0.8-3 μM, most preferably 0.9-2 μM, and even most preferably about 1 μM CHIR99021. It will be understood that an effective concentration of any receptor/enzyme agonist or inhibitor varies with the availability and biological activity of the respective compound. The serum-free supplement applied in step (i), (ii) and (iii) of the method is formulated to result in a final concentration of 0.5-50 mg/ml albumin (preferably 1-40 mg/ml, more preferably 2-30 mg/ml, still more preferably 3-20 mg/ml, most preferably 4-10 mg/ml, and even most preferably 4.5-7.5 mg/ml such as about 5 mg/ml), 1-100 μg/ml transferrin (preferably 2-90 μg/ml, more preferably 3-80 μg/ml, even more preferably 4-70 μg/ml, still more preferably 5-60 μg/ml, more preferably 6-50 μg/ml, more preferably 7-40 μg/ml, more preferably 8-30 μg/ml, more preferably 9-20 μg/ml, such as about 10 μg/ml),

0.1-10 μg/ml ethanol amine (preferably 0.2-9 μg/ml, more preferably 0.3-8 μg/ml, even more preferably 0.4-7 μg/ml, still more preferably 0.5-6 μg/ml, more preferably 0.6-5 μg/ml, more preferably 0.7-4 μg/ml, more preferably 0.8-3 μg/ml, more preferably 1-2.5 μg/ml, such as about 2 μg/ml),

0.003-0.3 μg/ml sodium selenite (preferably 0.005-0.2 μg/ml, more preferably 0.01-0.1 μg/ml, even more preferably 0.02-0.05 μg/ml, and most preferably about 0.03 μg/ml, such as about 0.032 μg/ml),

0.4-40 μg/ml L-Carnitine HCl (preferably 0.5-30 μg/ml, more preferably 1-20 μg/ml, even more preferably 2-10 μg/ml, most preferably 3-5 μg/ml, and even most preferably about 4 μg/ml),

0.1-10 μg/ml Hydrocortisone (preferably 0.2-9 μg/ml, more preferably 0.3-8 μg/ml, even more preferably 0.4-7 μg/ml, still more preferably 0.5-6 μg/ml, more preferably 0.6-5 μg/ml, more preferably 0.7-4 μg/ml, more preferably 0.8-3 μg/ml, more preferably 0.9-2 μg/ml, such as about 1 μg/ml),

0.05-5 μl/ml Fatty acid supplement (preferably 0.1-4 μl/ml, more preferably 0.2-3 μl/ml, even more preferably 0.3-3 μl/ml, most preferably 0.4-2 μl/ml, and even most preferably 0.45-1 μl/ml, such as about 0.5 μl/ml), and

0.0001-0.1 μg/ml triodo-L-thyronine (T3) (preferably 0.001-0.01 μg/ml, more preferably 0.002-0.0075 μg/ml, even more preferably 0.003-0.005 μg/ml, and most preferably about 0.004 μg/ml).

In addition, the serum-free supplement may further comprise one or more components selected from the group consisting of vitamin A, D-galactose, L-carnitine, linoleic acid, linolenic acid, progesterone, and putrescine. These components are conducive for the viability of the cells. Suitable concentrations of the respective components are known to the skilled person or can be easily determined using routine measures.

The serum-free supplement referred to in step (i) is also commercially available. For example, B27® supplement or B27® supplement minus insulin can be used. In a preferred embodiment, the B27® supplement or B27® supplement minus insulin used in step (i) of the above method is applied in an amount of 0.1-10% B27® or B27® minus insulin, preferably 0.5-8%, more preferably 1-6%, even more preferably 1.5-4%, and most preferably about 2% B27® or B27® minus insulin.

As demonstrated in the examples below, it has been proven advantageous to include an effective amount of ascorbic acid or a salt or a derivative thereof into the basal medium of step (i). In a preferred embodiment, the basal medium of step (i) comprises 10-1000 μM, preferably 50-400 μM, more preferably 100-300 μM, even more preferably 150-250 μM, and most preferably about 200 μM of ascorbic acid or a salt or a derivative thereof. The ascorbic acid may be delivered in the free form or as a salt. Since ascorbate is the active ingredient, any salt or derivative of ascorbic acid may be used, which provides the ascorbate to the cells, provided the counter ion has no detrimental effect on the cells. As shown in the examples, one suitable salt or derivative of ascorbic acid is ascorbate-2-phosphate.

The length of step (i) and the concentration of factors such as BMP4, Activin A, FGF2, and the GSK3-inhibitor may be optimized by monitoring the efficiency of induction of mesoderm differentiation. This can be achieved by monitoring the expression of cell surface or pluripotency markers, i.e. by (a) a decrease of TRA-1-60 and OCT4 positive cells (pluripotent stem cells) and (b) an increase of MIXL1 and Mesp1 positive cells (mesoderm) (see also FIG. 4f herein).

Briefly, cells are fixed using ethanol, blocked using standard protocols, and then stained with primary antibodies directed against TRA-1-60, OCT4, MIXL1 and/or Mesp1 (cf. Table 2 below) in blocking buffer for 45 min, optionally followed by secondary antibodies (if the primary antibody is not fluorescence labelled) in blocking buffer and Hoechst for 30 min at 4° C. (cf. Table 2 below). A BD LSRII is used for flow cytometry analysis (BD Biosystems). For live cells populations are gated based on forward-side scatter profiles. BD FACSDiva Software (BD Bioscience) or Cyflologic v1.2.1 (Cyflo Ltd) are used for analysis. Induction of mesoderm differentiation is indicated if

(a) less than 50%, preferably less than 40%, more preferably less than 30%, even more preferably less than 20%, most preferably less than 10%, and even most preferably less than 5% of the cells of the live cells population are positive for TRA-1-60; and/or less than 50%, preferably less than 40%, more preferably less than 30%, even more preferably less than 20%, most preferably less than 10%, and even most preferably less than 5% of the cells of the live cells population are positive for OCT4; and (b) more than 20%, preferably more than 30%, more preferably more than 40%, even more preferably more than 50%, and most preferably more than 60%, of the cells of the live cells population are positiv for MIXL1; and/or more than 20%, preferably more than 30%, more preferably more than 40%, even more preferably more than 50%, and most preferably more than 60% of the cells of the live cells population are positive for Mesp1.

Usually, step (i) is carried out for 48-96 h. Preferably, step (i) is carried out for 60-84 h, and more preferably step (i) is carried out for 66-78 h.

The basal medium used in step (ii), can be selected from DMEM/F12, StemPro, Iscove's medium, αMEM, DMEM, and RPMI. Preferably, the basal medium used in step (ii) is RPMI supplemented with pyruvate. However any suitable basal medium may be used in the method.

If deemed appropriate, the basal medium of step (ii) may be supplemented with non-essential amino acids. If αMEM is used as the basal medium in step (ii), the basal medium need not be supplemented additionally with non-essential amino acids. The non-essential amino acids are commercially available as a combined supplement. Such a supplement for example comprises 750 mg/L glycine, 890 mg/L L-alanine, 1320 mg/L L-asparagine, 1330 mg/L L-aspartic acid, 1470 mg/L L-glutamic acid, 1150 mg/L L-proline, and 1050 mg/L L-serine.

The basal medium in step (ii) may be independently selected from the basal medium applied in step (i). However, in a preferred embodiment, the basal medium in steps (i) and (ii) is the same.

The inhibitor of the Wnt-signaling pathway in the basal medium of step (ii) may be any inhibitor of the Wnt-signaling pathway, which can be suitably applied in the method of the invention. Preferably, said inhibitor of the Wnt-signaling pathway is selected from the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR3, IWR-4, IWR-5, XAV939, DKK1, quercetin, ICG-001, pyrvinium, CCT031374, iCRT3, 5, 14, CPG049090, and NC043. More preferably said inhibitor of the Wnt-signaling pathway is selected from the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR-3, IWR-4, IWR-5, XAV939, DKK1. As demonstrated in the examples below, one particularly useful inhibitor of the Wnt-signaling pathway in the basal medium of step (ii) is IWP4.

The serum-free supplement referred to in step (ii) is as defined for step (i) above. The serum-free supplements applied in step (i) and (ii) may be the same or not. Likewise, B27® supplement or B27® supplement minus insulin can be used in step (ii). In a preferred embodiment, the B27® supplement or B27® supplement minus insulin used in step (ii) of the above method is applied in an amount of 0.1-10% B27® or B27® minus insulin, preferably 0.5-8%, more preferably 1-6%, even more preferably 1.5-4%, and most preferably about 2% B27® or B27® minus insulin. It will be understood by the skilled person that the concentration of an effective amount of an inhibitor of the Wnt-signaling pathway varies with the availability and inhibition constant of the inhibitor in question. For example, in case of IWP4, the basal medium of step (ii) may comprise 0.1-10 μM IWP4, preferably 1-9 μM, more preferably 2-8 μM, even more preferably 3-7 μM, still more preferably 4-6 μM, and most preferably about 5 μM IWP4. It will be understood that an effective concentration of any receptor/enzyme agonist or inhibitor varies with the availability and biological activity of the respective compound.

As demonstrated in the examples below, it has been proven advantageous to include an effective amount of ascorbic acid or a salt or a derivative thereof into the basal medium of step (ii). In a preferred embodiment, the basal medium of step (ii) comprises 10-1000 μM, preferably 50-400 μM, more preferably 100-300 μM, even more preferably 150-250 μM, and most preferably about 200 μM of ascorbic acid or a salt or a derivative thereof. The ascorbic acid may be delivered in the free form or as a salt. Since ascorbate is the active ingredient, any salt or derivative of ascorbic acid may be used, which provides the ascorbate to the cells, provided the counter ion has no detrimental effect on the cells. As shown in the examples, one suitable salt or derivative of ascorbic acid for use in the basal medium in step (ii) is ascorbate-2-phosphate.

The length of step (ii) and the concentration of the remaining constituents such as the inhibitor of the Wnt-signaling pathway may be optimized by monitoring the efficiency of induction of cardiac differentiation of the cells. This can be achieved by monitoring the expression of differentiation markers, i.e. by an increase of Nkx2.5 and actinin.

Briefly, cells are fixed using ethanol, blocked, and then stained with primary antibodies directed against Nkx2.5 and/or actinin (cf. Table 2 below) in blocking buffer for 45 min, optionally followed by secondary antibodies (if the primary antibody is not fluorescence labelled) in blocking buffer and Hoechst for 30 min at 4° C. (cf. Table 2 below). A BD LSRII is used for flow cytometry analysis (BD Biosystems). For live cells populations are gated based on forward-side scatter profiles. BD FACSDiva Software (BD Bioscience) or Cyflologic v1.2.1 (Cyflo Ltd) are used for analysis. Induction of cardiac differentiation is indicated if more than 20%, preferably more than 30%, more preferably more than 40%, even more preferably more than 50%, and most preferably more than 60%, of the cells of the live cells population are positiv for Nkx2.5; and/or more than 20%, preferably more than 30%, more preferably more than 40%, even more preferably more than 50%, and most preferably more than 60% of the cells of the live cells population are positive for actinin (see also FIGS. 4d and 4f herein).

Usually, step (ii) is carried out for 8-12 days. Preferably, step (ii) is carried out for 9-11 days, and most preferably step (ii) is carried out for 10 days.

The basal medium used in step (iii), can be selected from DMEM/F12, StemPro, Iscove's medium, αMEM, DMEM, and RPMI. Preferably, the basal medium used in step (iii) is RPMI supplemented with pyruvate. However any suitable basal medium may be used in the method.

If deemed appropriate, the basal medium of step (iii) may be supplemented with non-essential amino acids. If αMEM is used as the basal medium in step (iii), the basal medium need not be supplemented additionally with non-essential amino acids. The non-essential amino acids are commercially available as a combined supplement. Such a supplement for example comprises 750 mg/L glycine, 890 mg/L L-alanine, 1320 mg/L L-asparagine, 1330 mg/L L-aspartic acid, 1470 mg/L L-glutamic acid, 1150 mg/L L-proline, and 1050 mg/L L-serine.

The basal medium in step (iii) may be independently selected from the basal medium applied in steps (i) and/or (ii). However, in a preferred embodiment, the basal medium in steps (ii) and (iii) is the same. More preferably, the basal medium in steps (i), (ii) and (iii) is the same.

As demonstrated in the examples below, it has been proven advantageous to include an effective amount of ascorbic acid or a salt or a derivative thereof into the basal medium of step (iii). In a preferred embodiment, the basal medium of step (iii) comprises 10-1000 μM, preferably 50-400 μM, more preferably 100-300 μM, even more preferably 150-250 μM, and most preferably about 200 μM of ascorbic acid or a salt or a derivative thereof. The ascorbic acid may be delivered in the free form or as a salt. Since ascorbate is the active ingredient, any salt or derivative of ascorbic acid may be used, which provides the ascorbate to the cells, provided the counter ion has no detrimental effect on the cells. As shown in the examples, one suitable salt or derivative of ascorbic acid for use in the basal medium in step (iii) is ascorbate-2-phosphate.

The serum-free supplement referred to in step (iii) is a serum-free supplement as defined for step (i) above. The serum-free supplements applied in steps (i), (ii) and (iii) may be the same or not. Likewise, B27® supplement or B27® supplement minus insulin can be used in step (iii). In a preferred embodiment, the B27® supplement or B27® supplement minus insulin used in step (iii) of the above method is applied in an amount of 0.1-10% B27® or B27® minus insulin, preferably 0.5-8%, more preferably 1-6%, even more preferably 1.5-4%, and most preferably about 2% B27® or B27® minus insulin.

The basal medium of step (iii) further comprises an effective amount of TGFβ1. For example, the basal medium of step (iii) may comprise 0.1-10 ng/ml TGFβ1, preferably 0.2-9 ng/ml, more preferably 0.3-8 ng/ml, even more preferably 0.4-7 ng/ml, still more preferably 0.5-6 ng/ml, more preferably 0.6-5 ng/ml, more preferably 0.7-4 ng/ml, more preferably 0.8-3 ng/ml, most preferably 0.9-2 ng/ml, and even most preferably about 1 ng/ml TGFβ1.

As shown in the examples, it is advantageous for cardiac maturation if the basal medium of step (iii) does not comprise an effective amount of FGF2. In contrast thereto, calcium has been shown to improve cardiac maturation. Accordingly, in a preferred embodiment, the basal medium of step (iii) comprises 0.5-3 mM Ca²⁺, preferably 0.5-2.75 mM Ca²⁺, more preferably 1-2.25 mM Ca²⁺, even more preferably 1-1.5 mM mM Ca²⁺, and most preferably about 1.2 mM Ca²⁺.

Usually, step (iii) of the method of the invention is carried out under mechanical stimulation, e.g. on a stretch device, as generally known in the art. Preferably, the stretch device applies a static, phasic or dynamic stretch to the BHM. More specifically, mechanical stretching can be (a) static, (b) dynamic, or (c) flexible against a resilient load. Preferably, the mechanical stimulation in step (iii) is dynamic mechanical stimulation or static stretch. In a more preferred embodiment, the mechanical stimulation in step (iii) is dynamic mechanical stimulation against a resilient load to facilitate auxotonic contractions.

Whether cardiac maturation is promoted can be tested by optical inspection for spontaneous or electrically stimulated contractions. Preferably, cardiac maturation is monitored by an isometric contraction experiment, wherein a twitch force development of >0.01 mN is indicative for cardiac maturation.

Briefly, contraction experiments are performed in organ baths at 37° C. under constant bubbling with 5% CO₂ and 95% O₂ to maintain a physiological pH in Tyrode's solution containing (all in mM): 120 NaCl, 1 MgCL₂, 0.2 CaCl₂, 5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucose and 0.56 ascorbate. Calcium is adjusted using a 0.2 M calcium chloride solution. All BHM are analysed at 3 Hz with 5 ms square pulses of 200 mA electrical current in order to pace at approximately the embryonic heart rate. Stimulation frequency is altered to confirm proper force-frequency response (Bowditch mechanism). BHM are mechanically stretched at intervals of 125 μm until the maximum twitch force is observed (force-length response; Frank-Starling mechanism).

Usually, step (iii) is carried out for at least 72 h. Although there is no particular upper limit for the length of step (iii), said step is usually carried out for less than 100 days. In specific embodiments, step (iii) may be carried out for 4-50 days, such as for about 15 days.

Step (i) of the method of the invention may be preceded by a seeding step, wherein said pluripotent stem cells are seeded in a ratio of (2.5-6×10⁶ cells/1 mg collagen)/1 ml medium in a suitable mould. Preferably, the seeding step is carried out 18-30 h prior to step (i).

The medium used in the seeding step usually comprises 0.2-2 mg/ml collagen (preferably 0.3-1.9 mg/ml, more preferably 0.4-1.8 mg/ml, even more preferably 0.4-1.7 mg/ml, still more preferably 0.5-1.6 mg/ml, more preferably 0.6-1.5 mg/ml, more preferably 0.7-1.4 mg/ml, more preferably 0.8-1.3 mg/ml, more preferably 0.9-1.2 mg/ml, such as about 1 mg/ml). The collagen is preferably of medical grade and selected from the group consisting of collagen type I, collagen type III, collagen type V, and a mixture thereof. In a more preferred embodiment, at least 90% of said collagen collagen type I. However, said collagen may also further comprises one or more extracellular matrix components selected from the group consisting of elastin, laminin, entactin, nidogen, proteoglycan, and fibronectin. Usually, the exact composition of the collagen will depend on the origin, from where it is derived from. The collagen is preferably of human origin, but bovine or porcine origin, or marine origin, such as from algae or fish origin, is also contemplated. Alternatively, recombinant collagen may also be used.

In order to achieve suitable cell densities, for some pluripotent cell lines it may be helpful to supplement the medium used in the seeding step with a ROCK-inhibitor. Therefore, in a preferred embodiment, the medium used in the seeding step further comprises a ROCK-inhibitor. The ROCK-inhibitor may be any ROCK-inhibitor, which can be suitably applied in the method of the invention. Preferably, said ROCK inhibitor is selected from Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil, GSK429286A, and RKI-1447, preferably selected from Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil, and more preferably the ROCK inhibitor is selected from Y27632 or H-1152P. As demonstrated in the examples below, one particularly useful ROCK-inhibitor is Y27632.

It will be understood by the skilled person that the concentration of an effective amount of a ROCK-inhibitor varies with the availability and inhibition constant of the inhibitor in question. For example, in case of Y27632, the medium used in the seeding step may comprise 1-50 μM, preferably 2.5-40 μM, more preferably 5-30 μM, even more preferably 7.5-20 μM, most preferably 8-12 μM, and most preferably about 10 μM Y27632.

It will be understood that an effective concentration of any receptor/enzyme agonist or inhibitor varies with the availability and biological activity of the respective compound.

Apart from the above disclosed method, the invention further relates to a BHM produced by said method. Despite the increased maturity observed in our BHM protocol, it should also be noted that the BHM is still a relatively immature tissue. Compared to adult heart tissue the BHM still has an inferior β-MHC/α-MHC ratio, and low but still retained expression of progenitor genes (e.g. ISL1). However, prolonged culture under appropriate culture conditions with biophysical stimulation may further increase maturity. There is already morphological evidence suggesting that this may also be the case in the BHM system.

The BHM obtained by the method disclosed herein exhibits the following characteristics: It can be paced at multiple frequencies up to at least 3 Hz, exhibits a calcium EC₅₀ of higher than 0.2 mM being preferably in the physiological range 4-8 mM, and a twitch tension of more than 200 μN. The twitch tension is increased in response to increased resting length and resting tension. In response to 1 μM isoprenaline, the BHM exhibits an inotropic response of more than 40 μN under paced conditions at 0.6 mM calcium, preferably more than 45 μN, more preferably more than 50 μN. Briefly, all contraction experiments are performed in organ baths at 37° C. and physiological pH in Tyrode's solution containing (all in mM): 120 NaCl, 1 MgCl₂, 0.2 CaCl₂, 5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucose and 0.56 ascorbate. Calcium is adjusted using a 0.2 M calcium chloride solution. All BHM are analysed at 3 Hz with 5 ms square pulses of 200 mA electrical current in order to pace at approximately the embryonic heart rate. Stimulation frequency is altered to confirm proper force-frequency response (Bowditch mechanism). BHM are mechanically stretched at intervals of 125 μm until the maximum twitch force is observed at 2 mM calcium (Frank-Starling mechanism). Subsequently, BHM are subjected to different calcium concentrations (0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mM) and the twitch force is recorded. For isoprenaline experiments the calcium concentration is adjusted to 0.6 mM and subsequently the isoprenaline concentration is adjusted to 1 μM.

Another characteristic of the BHM obtained by the method disclosed herein is that it comprises CD90⁺ stromal cells. Expression of CD90 can be determined using flow cystometry. Briefly, cells are fixed using ethanol, blocked, and then stained with primary antibodies directed against CD90 (cf. Table 2 below) in blocking buffer for 45 min, optionally followed by secondary antibodies (if the primary antibody is not fluorescence labelled) in blocking buffer and Hoechst for 30 min at 4° C. (cf. Table 2 below). A BD LSRII is used for flow cytometry analysis (BD Biosystems). For live cells populations are gated based on forward-side scatter profiles. BD FACSDiva Software (BD Bioscience) or Cyflologic v1.2.1 (Cyflo Ltd) are used for analysis.

The BHM may provide a good model system for investigating mechanisms driving maturation in a serum-free environment, and we have already demonstrated that with increased culture periods maturity may be increased (we showed increase isoprenaline sensitivity and improve tissue morphology). The capability of long term BHM culture without loss of function (at least 63 days) also suggests that long term pharmacological safety and efficacy experiments are possible. Hence, in a preferred embodiment, the BHM obtained by the method disclosed herein can be maintained for at least 63 days.

Using the traditional approach of differentiation followed by tissue engineering the differentiation cultures require extensive digestion protocols in order to yield single cell/small clumps required for cardiac tissue engineering applications. These digestion protocols destroy the extracellular environment and spatial distribution formed during development and may hence have a difficult to control inhibitory effect on the cardiac differentiation protocol.

Using the BHM as a model we demonstrated that factors affecting early development (ASC-2-P, dorsomorphin) and later development (mechanical stimulation, FGF2, TGFβ1, and calcium concentration) had a profound impact on BHM function and properties. Therefore, our BHM protocol may be a useful tool in the study of developmental processes not only governing cardiogenesis, but also tissue formation and properties.

Accordingly, the BHM obtained by the method disclosed herein can be suitably used as a research tool. For example, the use of the BHM obtained by the method disclosed herein in an in vitro-model for drug toxicity screening is contemplated. In other words, a method for screening drug toxicity is contemplated, comprising the step of contacting a BHM obtained by the method disclosed herein with a drug to be screened. Alternatively, the BHM obtained by the method disclosed herein may be used in an in vitro method for testing of cardiac function modulation by pharmacological candidate agents. Thus, also described is a method for testing of cardiac function modulation, comprising the step of contacting a BHM according to the invention with a pharmacological candidate agent.

Finally, the BHM obtained by the method disclosed herein can be used in medicine. Merely as an example, it is contemplated that the BHM obtained by the method disclosed herein can be advantageously used in heart repair.

The invention is further described by the following embodiments:

-   1. A method for producing bioengineered heart muscle from     pluripotent stem cells, comprising the steps of     -   (i) cultivating pluripotent stem cells in a basal medium         comprising an effective amount of (a) BMP4, Activin A, FGF2, a         GSK3-inhibitor, and (b) a serum-free supplement resulting in a         final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml         transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium         selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml         Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1         μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm         differentiation of said pluripotent stem cells;     -   (ii) cultivating the cells obtained in step (i) in a basal         medium comprising an effective amount of an inhibitor of the         Wnt-signaling pathway and a serum-free supplement as in (i),         thereby inducing cardiac differentiation of the cells; and     -   (iii) cultivating the cells obtained in step (ii) in a basal         medium comprising an effective amount of a serum-free supplement         as in (i), under mechanical stimulation, thereby promoting         cardiac maturation. -   2. The method of embodiment 1, wherein the pluripotent stem cells     are pluripotent stem cells of primate origin, preferably human     pluripotent stem cells. -   3. The method of embodiment 1 or 2, wherein the pluripotent stem     cells are selected from embryonic stem cells, induced pluripotent     stem cells, and parthenogenetic stem cells. -   4. The method of any one of embodiment 1-3, wherein the basal     medium, of step (i) comprises 10-1000 μM, preferably 50-400 μM, more     preferably 100-300 μM, even more preferably 150-250 μM, and most     preferably about 200 μM of ascorbic acid or a salt or a derivative     thereof. -   5. The method of embodiment 4, wherein the salt or derivative of     ascorbic acid is ascorbate-2-phosphate. -   6. The method of any one of embodiment 1-5, wherein step (i) is     carried out for 48-96 h, preferably wherein step (i) is carried out     for 60-84 h, most preferably wherein step (i) is carried out for     66-78 h. -   7. The method of any one of embodiment 1-6, wherein the basal medium     in step (i) comprises 1-20 ng/ml BMP4, preferably 2-15 ng/ml, more     preferably 2.5-10 ng/ml, more preferably 3-8 ng/ml, most preferably     4-6 ng/ml, and even most preferably about 5 ng/ml. -   8. The method of any one of embodiment 1-7, wherein the basal medium     in step (i) comprises 0.1-10 ng/ml FGF2, preferably 1-9 ng/ml, more     preferably 2-8 ng/ml, even more preferably 3-7 ng/ml, most     preferably 4-6 ng/ml, and even most preferably about 5 ng/ml. -   9. The method of any one of embodiment 1-8, wherein the basal medium     in step (i) comprises 1-20 ng/ml Activin A, preferably 2.5-18 ng/ml,     more preferably 5-16 ng/ml, even more preferably 7.5-14 ng/ml, still     more preferably 8-12 ng/ml, most preferably 8.5-10 ng/ml, and even     most preferably about 9 ng/ml. -   10. The method of any one of embodiment 1-9, wherein the     GSK3-inhibitor in the basal medium of step (i) is selected from the     group consisting of CHIR99021, CHIR98014, SB216763, TWS119,     Tideglusib, SB415286, and LY2090314. -   11. The method of embodiment 10, wherein the GSK3-inhibitor in the     basal medium of step (i) is CHIR99021. -   12. The method of embodiment 11, wherein the basal medium in     step (i) comprises 0.1-10 μM CHIR99021, preferably 0.2-9 μM, more     preferably 0.3-8 μM, even more preferably 0.4-7 μM, still more     preferably 0.5-6 μM, more preferably 0.6-5 μM, more preferably 0.7-4     μM, more preferably 0.8-3 μM, most preferably 0.9-2 μM, and even     most preferably about 1 μM CHIR99021. -   13. The method of any one of embodiment 1-12, wherein the serum-free     supplement in step (i) comprises 0.1-10% B27 or B27 minus insulin,     preferably 0.5-8%, more preferably 1-6%, even more preferably     1.5-4%, and most preferably about 2% B27 or B27 minus insulin. -   14. The method of any one of embodiment 1-13, wherein the basal     medium used in step (i), is DMEM/F12, StemPro, Iscove's medium,     αMEM, DMEM, and RPMI. -   15. The method of claim 14, wherein the basal medium used in     step (i) is RPMI supplemented with pyruvate. -   16. The method of any one of embodiment 1-15, wherein the inhibitor     of the Wnt-signaling pathway in the basal medium of step (ii) is     selected from the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3,     IWR-2, IWR-3, IWR-4, IWR-5, XAV939, DKK1, quercetin, ICG-001,     pyrvinium, CCT031374, iCRT-3, 5, 14, CPG049090, NC043; preferably     selected from the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3,     IWR-2, IWR-3, IWR-4, IWR-5, XAV939, DKK1. -   17. The method of embodiment 16, wherein the inhibitor of the     Wnt-signaling pathway in the basal medium of step (ii) is IWP4. -   18. The method of embodiment 17, wherein the basal medium of     step (ii) comprises 0.1-10 μM IWP4, preferably 1-9 μM, more     preferably 2-8 μM, even more preferably 3-7 μM, still more     preferably 4-6 μM, and most preferably about 5 μM IWP4. -   19. The method of any one of embodiment 1-18, wherein the basal     medium of step (ii) comprises 10-1000 μM, preferably 50-400 μM, more     preferably 100-300 μM, even more preferably 150-250 μM, and most     preferably about 200 μM of ascorbic acid or a salt or a derivative     thereof. -   20. The method of embodiment 19, wherein the salt or derivative of     ascorbic acid is ascorbate-2-phosphate. -   21. The method of any one of embodiment 1-20, wherein step (ii) is     carried out for 8-12 days, preferably wherein step (ii) is carried     out for 9-11 days, most preferably wherein step (ii) is carried out     for 10 days. -   22. The method of any one of embodiment 1-21, wherein the serum-free     supplement in step (ii) comprises 0.1-10% B27 or B27 minus insulin,     preferably 0.5-8%, more preferably 1-6%, even more preferably     1.5-4%, and most preferably about 2% B27 or B27 minus insulin. -   23. The method of any one of embodiment 1-22, wherein the basal     medium used in step (ii), is DMEM/F12, StemPro, Iscove's medium,     αMEM, DMEM, and RPMI. -   24. The method of embodiment 23, wherein the basal medium used in     step (ii) is RPMI supplemented with pyruvate. -   25. The method of any one of embodiment 1-24, wherein the basal     medium of step (iii) further comprises 10-1000 μM, preferably 50-400     μM, more preferably 100-300 μM, even more preferably 150-250 μM, and     most preferably about 200 μM of ascorbic acid or a salt or a     derivative thereof. -   26. The method of embodiment 25, wherein the salt or derivative of     ascorbic acid is ascorbate-2-phosphate. -   27. The method of any one of embodiment 1-26, wherein step (iii) is     carried out for at least 72 h, preferably wherein step (iii) is     carried out for less than 100 days, more preferably wherein     step (iii) is carried out for 4-50 days, and most preferably wherein     step (iii) is carried out for about 15 days. -   28. The method of any one of embodiment 1-27, wherein the serum-free     supplement in step (iii) comprises 0.1-10% B27 or B27 minus insulin,     preferably 0.5-8%, more preferably 1-6% 7, even more preferably     1.5-4%, and most preferably about 2% B27 or B27 minus insulin. -   29. The method of any one of embodiment 1-28, wherein the basal     medium used in step (iii), is DMEM/F12, StemPro, Iscove's medium,     αMEM, DMEM, and RPMI. -   30. The method of embodiment 29, wherein the basal medium used in     step (i) is RPMI supplemented with pyruvate. -   31. The method of any one of embodiment 1-30, wherein the basal     medium of step (iii) further comprises 0.1-10 ng/ml TGF931,     preferably 0.2-9 ng/ml, more preferably 0.3-8 ng/ml, even more     preferably 0.4-7 ng/ml, still more preferably 0.5-6 ng/ml, more     preferably 0.6-5 ng/ml, more preferably 0.7-4 ng/ml, more preferably     0.8-3 ng/ml, most preferably 0.9-2 ng/ml, and even most preferably     about 1 ng/ml TGFβ1. -   32. The method of any one of embodiment 1-31, wherein the basal     medium of step (iii) does not comprise an effective amount of FGF2. -   33. The method of any one of embodiment 1-32, wherein the basal     medium of step (iii) comprises 0.5-3 mM Ca²⁺, preferably 0.5-2.75 mM     Ca²⁺, more preferably 1-2.25 mM Ca²⁺, even more preferably 1-1.5 mM     mM Ca²⁺, and most preferably about 1.2 mM Ca²⁺. -   34. The method of any one of embodiment 1-33, wherein the mechanical     stimulation in step (iii) is dynamic mechanical stimulation or     static stretch. -   35. The method of embodiment 34, wherein the mechanical stimulation     in step (iii) dynamic mechanical stimulation. -   36. The method of any one of embodiment 1-35, comprising prior to     step (i) a seeding step, wherein said pluripotent stem cells are     seeded in a ratio of (2.5-6×10⁶ cells/1 mg collagen)/1 ml medium in     a suitable mould. -   37. The method of embodiment 36, wherein the collagen is collagen I. -   38. The method of any one of embodiment 36-37, wherein said collagen     is of human origin, bovine origin, or marine origin. -   39. The method of any one of embodiment 36-38, wherein the medium     used in the seeding step further comprises a ROCK-inhibitor. -   40. The method of embodiment 39, wherein the ROCK inhibitor is     selected from Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil,     GSK429286A, and RKI1447, preferably selected from Y27632, H-1152P,     Thiazovivin, Fasudil, Hydroxyfasudil, and more preferably the ROCK     inhibitor is Y27632 or H-1152P, and most preferably the ROCK     inhibitor is Y27632. -   41. The method of embodiment 40, wherein the medium used in the     seeding step comprises 1-50 μM, preferably 2.5-40 μM, more     preferably 5-30 μM, even more preferably 7.5-20 μM, most preferably     8-12 μM, and most preferably about 10 μM Y27632. -   42. The method of any one of embodiment 36-41, wherein the seeding     step is carried out 18-30 h prior to step (i). -   43. A bioengineered human myocardium (BHM) produced by the method     according to any one of embodiment 1-42. -   44. The BHM of embodiment 43, which can be paced at multiple     frequencies of up at least 3 Hz. -   45. The BHM of embodiment 43 or 44, which exhibits an increased     twitch tension in response to increased resting length and resting     tension. -   46. The BHM of any one of embodiment 43-45, which exhibits a calcium     EC₅₀ higher than 0.2 mM. -   47. The BHM of any one of embodiment 43-46, which exhibits a twitch     tension of more than 200 μN. -   48. The BHM of any one of embodiment 43-47, which exhibits an     inotropic response to 1 μM isoprenaline of more than 40 μN under     paced conditions at 0.6 mM calcium, preferably more than 45 μN, more     preferably more than 50 μN. -   49. The BHM of any one of embodiment 43-48, which comprises     cardiomyocytes and CD90⁺ stromal cells. -   50. The BHM of any one of embodiment 43-49, which can be maintained     for at least 62 days. -   51. Use of the bioengineered human myocardium (BHM) according to any     one of embodiment 43-50 in an in vitro-model for drug toxicity     screening. -   52. Use of the bioengineered human myocardium (BHM) according to any     one of embodiment 43-50 in an in vitro method for testing of cardiac     function modulation by pharmacological candidate agents. -   53. Use of the bioengineered human myocardium (BHM) according to any     one of embodiment 43-50 as a research tool. -   54. The bioengineered human myocardium (BHM) according to any one of     embodiment 43-50 for use in medicine. -   55. The bioengineered human myocardium (BHM) according to any one of     embodiment 43-50 for use in heart repair.

DESCRIPTION OF THE FIGURES

FIG. 1: Optimization of early cardiac differentiation for robust and efficient cardiac differentiation. (A) Schematic of the developed cardiac differentiation protocol. (B,C) Effect of FGF-2 addition on cardiac differentiation in 2D culture. (D,E) Effect of varying BMP4 concentration on cardiac differentiation in 2D culture whilst CHIR is present. (F,G) Effect of removing each factor individually from the 2D cardiac differentiation protocol, all factors were added daily from days 0-3 except IWP4 which was added every 2-3 days from days 3-13. (H, I, J) Assay for the presence of contaminating cell types using qPCR. (K) Immuno-staining for cardiomyocyte markers. (L) Flow cytometry for cardiomyocytes (n=6 experiments). (M) Immuno-staining for stromal cell markers. (N) Flow cytometry for stromal cells (n=6 experiments). All data is n=3 experiments unless otherwise noted. qPCR data (MESP-1, OCT4, SOX17 and NEUROD1) is normalized to GAPDH. * Indicates statistically significant difference (P<0.05) using ANOVA with Tukey's Multiple Comparison Post Hoc test. **Indicates statistically significant difference from samples supplemented with no factors. ***Indicates statistically significant difference from samples supplemented with no factors, all factors minus BMP4, all factors minus ACT-A and all factors minus IWP-4.

FIG. 2: BHM can be formed directly from hPSC. (A) BHM at 22 days of differentiation. (B) Whole-mount immunostaining. (C) Isometric twitch tension (force of contraction) in response to varying calcium concentration, n=7 from 4 experiments. (D) Flow cytometry profiles of pluripotency (TRA-1-60/OCT4) and cardiac markers (α-actinin), n=3-4 experiments. (E) Flow cytometry of stromal cell markers at day 22, n=3 experiments. (F) qPCR expression profiles of markers for pluripotency, mesodermal differentiation, and cardiac differentiation; data is normalized to GAPDH expression, n=3 experiments. *Indicates statistically significant difference (P<0.05) compared to −1 days using ANOVA with Tukey's Multiple Comparison Post Hoc test.

FIG. 3: Optimization of BHM culture condition reveals different parameters respond specifically to different stimuli. (A) ASC-2-P supplementation improves BHM, isometric twitch tension (force of contraction) in response to varying calcium concentration, n=8-9 from 3 experiments, *indicates statistically significant difference (P<0.05) compared to control using two-way ANOVA with Bonferroni post hoc tests. (B) Flow cytometry analysis of ASC-2-P experiments for cardiomyocyte and stromal cell markers, n=7-8 from 3 experiments. (C) Mechanical stimulation improves BHM function, isometric twitch tension (force of contraction) in response to varying calcium concentration, n=9-11 from 4 experiments, *indicates statistically significant difference (P<0.05) for both mechanical stimulation regimes compared to control using two-way ANOVA Bonferroni post hoc tests. (D) Mechanical stimulation devices. (E) Whole-mount immuno-staining of BHM under control and mechanical stimulation regimes. (F) Growth factors (FGF2:10 ng/mL and TGFb1:1 ng/mL) added during cardiac maturation regulate BHM function, isometric twitch tension (force of contraction) in response to varying calcium concentration, n=9-11 from 4 experiments. (G) Cardiomyocyte cell size analysis for the growth factor experiments using flow cytometry, n=6 from 3 experiments, *indicates statistically significant difference (P<0.05) compared to control using Student's t-test. (H) qPCR expression of β-MHC/α-MHC ratio for the growth factor experiments, n=3-6 experiments, *indicates statistically significant difference (P<0.05) compared to control using ANOVA with Tukey's Multiple Comparison Post Hoc test. (I) qPCR expression of ANP and Sk Act for the growth factor experiments, n=3 experiments. (3) Adjusting calcium to 1.2 mmol/L during cariac maturation improves BHM function, isometric twitch tension (force of contraction) in response to varying calcium concentration, n=10-11 from 4 experiments, *indicates statistically significant difference (P<0.05) compared to control using two-way ANOVA Bonferroni post hoc tests. (K) resting tension of BHM for the calcium experiments, n=10-11 from 4 experiments. (L) Elastic modulus of BHM for the calcium experiments, n=10-11 from 4 experiments, *indicates statistically significant difference (P<0.05) compared to control using ANOVA with Tukey's Multiple Comparison Post Hoc test.

FIG. 4: BHM produced using the optimized protocol exhibit in-vivo-like properties. (A) BHM can be paced electrically at various rates. (B) BHM respond with increased twitch tension (force of contraction) to increased length (Frank-Starling mechanism). (C) Twitch tension (force of contraction) comparison of 22 day old (from previous data) and 29-30 day old BHM, n=11 from 4 experiments for 22 days and n=7 from 2 experiments for 29-30 days. (D) Inotropic response of 22 day old BHM to isoprenaline (1 μmol/L) under paced conditions at 0.6 mM calcium. (E) Inotropic response of 29-30 day old BHM to isoprenaline (1 μmol/L) under paced conditions at 0.6 mM calcium. (F) Comparison of inotropic response to isoprenaline (1 μmol/L) with age, n=11 from 4 experiments for 22 days and n=7 from 2 experiments for 29-30 days, *indicates statistically significant difference (p<0.05) using Student's t-test.

FIG. 5: The BHM protocol can be used in all tested PSC lines. (A, B, C) Data from HES3-BHM; (D, E, F) Data from hIPS-G1-BHM. (A,D) Isometric twitch tension (force of contraction) in response to varying calcium concentration, n=4 for each line. (B,E) Whole-mount immuno-staining. (C,F) Flow cytometry analysis of cardimoycytes and stromal cells, n=3 per line.

FIG. 6: 2D cardiac differentiation of multiple hPSC lines. Flow cytometry analysis of cardiac (α-actinin, SIRPA) and stromal cell markers (PDGFRα, α-SMA, collagen I).

FIG. 7: The BHM can be used to model developmental processes, using BMP signalling inhibition as an example. (A) qPCR analysis of multiple markers at day 13 of BHM formation. (B) Cell cycle analysis using flow cytometry gating on α-actinin+ cells. (C) Cardiomyocyte number per BHM. (D) Isometric twitch tension (force of contraction) in response to varying calcium concentrations, *indicates statistically significant difference (P<0.05) compared to control using A+B) Student's t-test (n=3-4) or D) two-way ANOVA Sidak's Multiple Comparison post test.

FIG. 8: Summary of protocols used for 2D cardiac differentiation and BHM formation. (A) Protocol used for experiments shown in FIG. 1. (B) Protocol used for experiments shown in FIG. 2. (C) Protocol used for experiments shown in FIG. 3—addition of ascorbic acid. (D) Protocol used for experiments shown in FIG. 3—mechanical stimulation and growth factors. (E) Protocol used for experiments shown in FIG. 3—addition of calcium. (F) Protocol used for experiments shown in FIG. 5. (G) Protocol used for experiments shown in FIG. 6. (H) Protocol used for experiments shown in FIG. 7.

FIG. 9: Custom-made supplement to replace B27®. (A) Contractile force of BHMs (hES2) made with B27® or custom-made supplement (CMS) at 2 mM extracellular calcium, n=2/group. (B) Total CM number in BHM made with B27® or custom-made supplement (CMS), n=2/group.

The following examples are meant to further illustrate, but not limit the invention. The examples comprise various technical features, and it will be appreciated that the invention also relates to combinations of the technical features presented in this exemplifying section.

FIG. 10: Supplementation of the culture medium with TGFß-1 during the cardiac maturation phase enhances contractile function of BHM (FOC: force of contraction) in a concentration dependent manner (0.3-10 ng/ml tested; n=5-7BHMs/condition).

EXAMPLES

Cardiac Differentiation Requires Optimization of Early Cardiac Mesoderm Induction

It has been demonstrated that non-myocyte cell fractions or stromal cells are essential for the function of engineered heart tissues. For this reason a cardiac differentiation protocol was firstly required which consistently produced cardiomyocytes and fibroblasts/stromal cells. The inventors optimized their cardiac differentiation protocol (FIG. 1a ) for both yield and consistency, based on a previously published serum-free 2D hPSC differentiation protocol (Hudson et al. Stem Cells Dev 21, 1513-1523 (2012)). It was reasoned that robustness and efficiency could be enhanced if WNT activity was stabilized during the mesoderm induction phase. As surrogate marker for mesoderm induction MESP1 expression was analysed by qPCR on culture day 3; this was followed by flow cytometry for α-actinin (cardiomyocyte marker) at day 16, which were found correlated very well with the amount of beating activity. The most important steps for the progression from the previously published protocol to the new protocol are summarised in FIG. 1.

The earlier protocol used cardiac mesoderm induction with BMP4 and Activin-A for the first 3 days followed by cardiac specification using a WNT inhibitor IWP4 (Hudson et al Stem Cells Dev 21, 1513-1523 (2012)). Consistent with the essential requirement of FGF2 for early mesoderm formation in recent studies using hPSCs and in vivo development, the addition of 5 ng/ml FGF2 in during the first 3 days of differentiation resulted in a trend to increased MESP1 expression (FIG. 1b ) and subsequently a trend to increased α-actinin (FIG. 1c ). However, it was enforced WNT signalling that helped improve consistency and differentiation, consistent with its essential role in induction of early mesoderm. To enforce WNT signalling CHIR99021 was used, a small molecule inhibitor of GSK3β which induces WNT signalling even in the presence of canonical WNT inhibitors. CHIR alone or with the differentiation factors used in FIG. 1b,c was unable to induce any beating activity. When the BMP4 concentration was varied, optimal and consistent induction of MESP1 (FIG. 1d ) and α-actinin expression (FIG. 1e ) was found at a BMP4 concentration of 5 ng/ml. Each factor from the differentiation protocol was then individually removed to demonstrate its requirement in the efficient and consistent induction of MESP1 (FIG. 1f ) and subsequently α-actinin expression (FIG. 1g ).

In order to form BHM it is also important that stromal cell populations are present. It was therefore investigated whether stromal cells or other potentially contaminating cell types were present in the optimized differentiation protocol. Very low levels of potentially contaminating cell populations were found, analysed using qPCR for hPSCs (OCT4 also known as POU5F1) (FIG. 1h ), endoderm (SOX17) (FIG. 1i ), neural (NEUROD1) (FIG. 1j ), and early mesoderm (MESP) (data not shown). Very high expression of NA2-5 and β-MHC (also known as MYH7) were found in the cardiac differentiation culture of the invention compared to the other conditions (with no factors or without IWP4, data not shown). Additionally it was found that both cardiomyocytes (FIG. 1k -1) and different stromal cell types, including: α-smooth muscle actin positive cells (α-SMA⁺), collagen I positive cells (COLI ⁺) cells, and α-SMA⁺ COLI ⁺ cells were present (FIG. 1m,n ). Together this data suggests that the cardiac differentiation protocol of the invention efficiently produces cardiomyocytes with the rest of the cells being predominantly stromal cells, thus providing the required cell composition for tissue engineering applications.

Directed Formation of BHM

After optimization of the cardiac differentiation protocol the hypothesis was tested whether one could form BHM directly from hPSCs. The new serum-free cardiac differentiation protocol (FIG. 1) was used with an additional maturation step where the rings were removed from the molds and put onto static stretchers (+10% of slack length) in medium containing 5 ng/ml FGF2 and 200 μM ascorbate-2-phosphate (ASC-2-P). It was found that this protocol was effective in forming BHM (FIG. 2a ). The BHM started to contract spontaneously in different areas by day 13 and by day 15-17 the contractions became synchronous and rhythmic, and continued until analysis at day 22 (data not shown). The cardiomyocytes in the BHM had an elongated and striated morphology (FIG. 2b ) and the BHM could be electrically paced and had a measurable force of contraction with responsiveness to calcium concentration (FIG. 2c ).

The development of the BHM followed known developmental pathways. The hPSCs were largely differentiated by day 3 indicated by a decrease in TRA-1-60⁺/OCT4⁺ cells and OCT4 expression (FIG. 2d,f ) and a concomitant expression of early mesodermal markers MIXL1 and MESP1 (FIG. 2f ). By day 8 there was a loss of MIXL1 and MESP1 expression (FIG. 2f ), with a concurrent increase in α-actinin⁺ cells (FIG. 2d ) and the expression of cardiomyocyte progenitor cell markers (FIG. 2f ). There were peaks in the expression of multiple transcription factors involved in cardiogenesis, including: TBX5 (peak at day 13), ISL1 (peak at day 8), and NKX2-5 (peak at day 13) (FIG. 2f ). This was followed by the expression of more mature cardiac markers α-MHC (also known as MYH6), β-MHC, ANP (also known as NPPA), and MLC2v (also known as MYL2) (FIG. 2f ). Interestingly, the expression of α-MHC peaked at day 13, followed by a large increase in β-MHC by day 22 and hence an increased β-MHC/α-MHC ratio (FIG. 2f ). Furthermore, there was little expression of endodermal and neural markers at day 22 (FIG. 2f ). Together this data suggests not only that the development of the BHM followed known developmental pathways, but also that cardiac maturation occurs as indicated by the drop in progenitor gene expression, increased β-MHC expression and β-MHC/α-MHC expression ratio, and increased expression of MLC2v (indicates maturity, see Tiburcy et al. Circ Res 109, 1105-1114 (2011)). Additionally, at day 22 it was found that the BHM was comprised of 30±6% (n=4 experiments) cardiomyocytes and a large proportion of stromal cells (FIG. 2d ). Representative flow cytometry plots are shown in FIG. 5.

Optimization of BHM Functionality

While the BHM protocol outlined in FIG. 2 represents the first entirely serum-free process of hPSC expansion and formation bioengineered myocardium, the inventors hypothesized that optimization could generate tissues with higher functionality and greater consistency. For these experiments contractile strength (twitch tension/force of contraction) was used as a primary determinant of function as twitch tension depends on a wide variety of myocardial properties including: cardiomyocyte number and phenotype, fibroblast number and phenotype, tissue connectivity, ECM composition, cell-cell connectivity and ECM-cell connectivity. As secondary factors the inventors used, 1) resting tension, because it reflects stroma cell function and extracellular matrix biology, 2) cardiomyocyte size, because of stimuli such as pharmacological stimuli, and 3) cell composition (cardiomyocytes:stroma cells), which is an important determinant for contractile performance. The parameters which changed are shown in FIG. 3.

ASC-2-P Enhances BHM Functionality

Ascorbate (vitamin C) plays a major role in the proper synthesis of collagen and is an antioxidant. It was therefore hypothesized that ascorbate (in the more stable form of ASC-2-P) supplemented during early BHM culture, days 0-13 (it was already added during days 13-22 in FIG. 2), would have a positive influence on BHM functionality given the importance of collagens during development. It was found that ASC-2-P significantly improved BHM twitch tension/force of contraction (FIG. 3a ) and induced a trend to increased cardiomyocyte fraction with no change in the stromal cell fraction (FIG. 3b ). It was also found that supplementation of ASC-2-P during the entire differentiation protocol improved differentiation in the inventor's 2D protocol by significantly increasing the number of cells without changes in cardiac differentiation efficiency (data not shown). Therefore, in both 2D and BHM formats, ASC-2-P may have increased cell survival and/or progenitor proliferation as proposed in a recent study (Cao et al. Cell Res 22, 219-236 (2012)).

BHM Function is Dependent on Mechanical Stimulation Regime

Next, it was assessed how static stretch and dynamic mechanical stimulation influenced BHM function. The devices used for static stretch and dynamic mechanical stimulation are shown in FIG. 3d . Both static stretch and dynamic mechanical stimulation significantly increased twitch tension/force of contraction of the BHM and both the mechanical stimulation regimes resulted in a similar BHM twitch tension (FIG. 3c ). Both mechanical stimulation regimes improved the morphology of the cardiomyocytes in the BHM, causing compact, elongated and striated muscle bundles to form (FIG. 3d ). Dynamic mechanical stimulation were preferred to static stretch, because it facilitates auxotonic contractions (Zimmermann et al, Nat Med 12, 452-458 (2006)).

Cardiomyocyte Properties are Dependant on Exogenous Growth Factors

There was a trend to decreased twitch tension/force of contraction when adding FGF2 and a trend to increased twitch tension when adding TGFβ1 (FIG. 3f ). Therefore, in the previous experiments where FGF2 was added during cardiac maturation, this may have actually been detrimental BHM function. It was found that both FGF2 and TGFβ1 induced an increase in cardiomyocyte size (FIG. 3g ). The addition of TGFβ1 resulted in a more mature β-MHC/α-MHC expression ratio (human heart=9) (FIG. 3h ) while pathological hypertrophy markers ANP (also known as NPPA) was decreased (FIG. 3i ). The addition of FGF2 did not change β-MHC/α-MHC expression ratio (FIG. 3h ), but induced (variably) pathological hypertrophy marker ANP (FIG. 3i ). Together this indicates both factors induce hypertrophy, which is consistent with in vivo results. However, FGF2 may be considered as an inducer of pathological hypertrophy, while TGFβ1 may be considered as an inducer of physiological hypertrophy.

In a further set of experiments the inventors investigated if supplementation of the culture medium with increasing TGFß-1 during cardiac maturation phase has an influence in contractile function of BHM. The inventors observed an enhancement of contractile function of BHM in a concentration dependent manner (FIG. 10).

In the earlier experiments the inventors found a large reduction in α-smooth muscle actin and collagen I positive cells in the BHM when compared with the 2D protocol (FIG. 1n vs. FIG. 2e ). This may be a reflection of subtle differences in myofibroblast/fibroblast differentiation in 2D and BHM cultures. To more homogeneously detect cardiac-fibroblast-like populations in hPSC-derived 2D and BHM cardiac differentiation cultures antibodies against the canonical CD90 (also known as THY1) were employed in subsequent experiments.

Adjusting Extracellular Calcium to Physiological Concentrations Improved BHM Function

Calcium concentration is tightly regulated in human serum with physiological calcium concentrations of 2.25-2.75 mM and 1.0-1.2 mM, for total calcium and ionized calcium, respectively. Because the concentration of calcium in RPMI medium is quite low (0.42 mM) compared to physiological calcium it was therefore assessed whether adjustment of free calcium concentration improved both BHM maturation and functionality. Adjustment of calcium to 1.2 mM (using a 0.2 M CaCl₂ solution) greatly increased the twitch tension of the BHM (FIG. 3j ). Moreover, an increase in resting tension (FIG. 3k ) and elastic modulus (FIG. 3l ) was observed. Optimal elastic modus lus has been demonstrated to improve the mechanical work done by cardiomyocytes during contractions which may increase contraction force. The mechanism behind the increased elastic modulus in response to increased calcium is currently unknown. Importantly, it should be noted that there were no changes in calcium handling proteins that were assessed using qPCR (CASQ2, PLN, ATP2A2, and RYR2, data not shown, n=3 experiments).

BHM Produced Using the Optimized Protocol Display In-Vivo Like Properties

BHM contacted spontaneously and coherently, and could be paced electrically at multiple frequencies up to at least 3 Hz (FIG. 4a ), covering the range of beating frequency observed during heart development. BHM also increased twitch tension (force of contraction) in response to increased resting length (and resting tension) consistent with the Frank-Starling mechanism (FIG. 4b ). With increased culture time no change in BHM twitch tension compared to the previous data was found, but an increase in calcium EC₅₀ from 0.2 to 0.7 mmol/L was observed (FIG. 4c ). Extended culture also resuited in an increased inotropic response to 1 μM isoprenaline indicating improved maturity over traditional culture formats where no increase has been observed (FIG. 4d-f ). Using whole-mount immunostaining, it was found that the BHM also has both stromal cells and endothelial cells present in the muscle bundles (data not shown). Importantly BHM culture could also be maintained for long term (until at least day 63) under the maturation conditions with an observed improvement in morphological appearance (data not shown).

The Unmodified BHM Protocol Works on Multiple Human Pluripotent Stem Cell Lines

Next, it was demonstrated that the optimized BHM (FIG. 5) and 2D (FIG. 6) protocols work on multiple hPSC lines. For these analyses HES2, HES3 and hIPS-G1 lines (reprogrammed dental fibroblasts using vector-free Cytotune reprogramming kit) were used. It is important to note that for both the 2D and BHM protocols it was found that modification of the seeding cell number or the use of a Rho-associated protein kinase inhibitor (10 μM, Y-27632) was required to achieve similar cell densities in all lines after the initial 24 h seeding phase (data not shown). When using the required seeding protocol for a particular cell line the 2D and BHM protocols could be used un-modified for those lines.

It was found that both the HES3 line and the hIPS line produced BHM with a lower twitch tension (FIG. 5a,d ) compared to the HES2 line (FIG. 3j ). However, both the HES3 and hIPS BHM had similar morphologies (FIG. 5b,e ). The cardiomyocyte fractions for the HES3 BHM was similar (FIG. 5c ) and for the hIPS BHM was lower (FIG. 5f ), compared to the HES2 BHM. The decreased functionality in the BHM from these lines is most likely from a lower cell number in the HES3 BHM (0.50±0.03×10⁶ cells, n=3) and hIPS BHM (0.55±0.05×10⁶ cells, n=3), compared to the HES2 BHM (0.74±0.13×10⁶ cells, n=6 from 3 experiments). Therefore, care must be taken to exclude differences caused by changes in cell number and composition when assessing different cell lines rather than different treatments on the same line.

BHM as a Developmental Model Reveals that BMP Signalling is Required for Terminal Differentiation of Human Cardiomyocytes

Inhibition of BMP signalling is embryonically lethal even when effects are restricted (or at least partially restricted) to the developing heart using CRE driven by various genes (for a review please see Kruithof et al. Differentiation 84, 89-102 (2012)). In these studies there have been multiple processes ascribed to BMP signalling including structural defects, myocardial properties including trabeculae structure and wall thickness, and cell phenotype including dysregulation of progenitor genes and reduced epithelial-to-mesenchymal transformation (EMT). In order to determine the effect of BMP signalling purely on myocardial development without systemic influences and anatomical limitations, BHM was therefore deemed to be a good model system.

In these experiments 2 μmol/L of the BMP receptor signalling inhibitor dorsomorphin were added with each medium exchange from day 6 onwards. At day 13 the dorsomorphin treated BHM failed to down regulate ISL1 while the expression of other more mature cardiac markers NKX2-5 and α-MHC were not altered (FIG. 7a ). When investigating EMT associated genes, it was found that there was no alteration in CDH1, CHD2, SNAIL1 or TGFβ2 expression indicating that EMT or factors regulating EMT were not altered in BHM after 13 days (FIG. 7a ). Using flow cytometry after 22 days it was found that there were more cardiomyocytes in the active cell cycle in the dorsomorphin treated group (FIG. 7b ). However, it was found that treatment with dorsomorphin did not alter the cardiomyocyte number (FIG. 7c ). It was also found that the total cell number per BHM and the fractions of cardiomyocytes (α-actinin⁺) and stromal cells (CD90⁺) did not change in the dorsomorphin treated group (data not shown). Despite these similarities there was a large reduction in twitch tension/force of contraction to 47% of the control group in the dorsomorphin treated BHM (FIG. 7d ). Interestingly, there was neither a change in BHM responsiveness to isoprenaline nor a change in cardiomyocyte size in the dorsomorphin treated BHM (data not shown).

As increased cell cycle activity did not lead to increased cardiomyocyte number and reduced twitch tension, it was calculated whether oxygen concentration may limit cardiomyocyte number in the BHM. To determine if this was the case, mathematically modelled oxygen diffusion profiles were based on models reported in the literature and the parameters in the different BHM conditions (cell number, cardiomyocyte fraction and size). It was found that when using the parameters for the control BHM there was no hypoxic region, even if the number of cardiomyocytes increased to 125% (data not shown).

Together the present data suggests that BMP inhibition using dorsomorphin results in an increased proliferative state, a result which is consistent with mouse in vivo experiments. However, there was no increase in cardiomyocyte number indicating that either there is increased apoptosis or the cardioymocytes are bi-nucleating. Regardless of the mechanism, inhibition of BMP signalling resulted reproducibly in a tissue phenotype that produces less contractile force (and also lower force per cardiomyocyte) and inferior myocardial tissue.

Custom-Made Supplement to Replace B27®

BHMs were generated from undifferentiated hESC under serum-free conditions according to standard BHM protocol. The standard protocol includes B27® supplement. In this experiment B27® supplement was replaced by a defined, custom-made supplement (CMS, Table 4).

The results show that B27® can be replaced by CMS. The force is similar, also the number of cardiomyocytes generated within the BHM is comparable (FIG. 9).

CONCLUSION

Using directed differentiation of PSCs in collagen I hydrogels the present application demonstrates that it is possible to guide BHM assembly under serum-free conditions. BHM has multiple applications including pharmacological studies, study of developmental processes, heart maturation processes and also potential regenerative applications.

In these examples, the robustness of the newly developed protocol was demonstrated by using multiple culture formats and multiple lines for differentiation. It will however be noted that while the experiment-experiment differentiation efficiency was consistent, the efficiency did vary when different batches of reagents were used. It is therefore prudent to establish strict reagent quality control in order to produce BHM with consistent and defined properties for both in vitro and potential therapeutic applications.

Methods PSC Culture

HES2-ROSA26-RFP (Irion et al. Nat Biotechnol 25, 1477-1482 (2007)) cells were obtained from Gordon Keller and HES3 cells were obtained from Embryonic Stem Cell International (ESI, Singapore). hIPS were generated from human gingiva biopsy-derived fibroblasts using the Cytotune Reprogramming Kit (Applied Biosystems) following the manufacturer's instructions.

For IPS generation, 6 days after viral transduction fibroblasts were plated on irradiated mouse embryonic fibroblasts in fibroblast medium (DMEM high glucose, 2 mmol/L glutamine, 10% FBS (PAA), 100 IU/ml Penicillin, 100 μg/ml Streptomycin, all Gibco except where indicated). The next day the medium was exchanged to PSC-medium (Knock-out DMEM (Gibco) supplemented with 20% Knock-Out Serum Replacement (KSR, Gibco), 2 mmol/L glutamine, 100 IU/ml Penicillin, 100 μg/ml Streptomycin, 1% non-essential amino acids (Gibco), and 10 ng/mL FGF2 (Miltenyi Biotec)). Emerging iPS colonies were mechanically picked and expanded by weekly passaging using 1 mg/ml Collagenase NB6 (Cresent Chemical Company).

For experiments, hPSCs were single cell adapted and cultured on irradiated human foreskin fibroblasts (HFF) in PSC-medium with daily medium changes and weekly passaging using 3 min TrypLE (Gibco) treatment (Ellerstrom et al., Stem Cells 25, 1690-1696 (2007)). Before characterization or differentiation experiments, the hPSCs were plated on 1:30 Matrigel (Millipore) in PBS (Gibco) coated-plates, at 2.5×10⁴ cells/cm² for HES2 or 5×10⁴ cells/cm² for HES3 and hIPS lines, and cultured for 3 days in 1:1 PSC-medium minus FGF-2 and HFF-conditioned medium (HFF-CM—harvested from 5 day confluent irradiated HFF cultures) with 10 ng/mL FGF2. The hIPS line also received 10 μmol/L Y-27632 (Stemgent). hPSCs were harvested for experiments by passaging using 3 min TrypLE treatment and then cultured in the appropriate format.

Pluripotent stem cell lines were regularly tested for mycoplasma using a test kit (Lonza) and characterized using standard assays. Pluripotency markers were assessed via PCR (endogenous OCT4, SOX2, KLF4, MYC), qPCR (OCT4, NANOG, REX1, DNMT3B) and immunostaining (OCT4, NANOG, TRA-1-60) (Chan et al. Nat Biotechnol 27, 1033-1037 (2009)). Demethylation of the OCT4 promoter was confirmed via bisulfite sequencing (Freberg et al. Mol Biol Cell 18, 1543-1553 (2007)). Karyotyping was used to determine if there were any genetic abnormalities (Campos et al. J Vis Exp 4 (2009)). Pluripotency was confirmed via teratoma formation in SCID mice via flank injection of 4-6×10⁶ cells.

Differentiation Medium

For differentiation experiments the hPSCs were then cultured in RPMI 1640 supplemented with 1 mmol/L sodium pyruvate, 100 IU Penicillin, 100 μg/ml Streptomycin and 2% B27 supplement (SF medium, all Gibco) and various factors as indicated. Factors used in this study included: L-ascorbic acid 2 phosphate sesquimagnesium salt hydrate (Sigma), BMP4 (R&D Systems), Activin A (R&D Systems), FGF2 (Miltenyi Biotec), dorsomorphin (Stemgent), CHIR99021 (Stemgent), IWP4 (Stemgent), and TGFβ1 (Peprotech).

2D Cardiac Differentiation

Cardiac differentiation was optimized on the HES2 line. HES2 hPSCs were plated at 5×10⁴ cells/cm² (1×10⁵ cells/cm² for HES3 and hIPS lines) on 1:30 Matrigel/PBS coated-plates and cultured in 1:1 PSC-medium minus FGF-2 and HFF-conditioned medium (HFF-CM—harvested from 5 day confluent irradiated HFF cultures) with 10 ng/mL FGF2. For the hIPS line 10 μM Y-27632 was added to this medium. After 1 day the cells were rinsed with RPMI medium, then differentiated as indicated in each figure with 0.5 ml of medium in each well of a 24 well plate. The protocol details for each figure are outlined in FIG. 8.

BHM Formation

BHM formation was optimized on the HES2 line. HES2 hPSCs were suspended 1:1 in PSC-medium minus FGF-2 and HFF-conditioned medium (HFF-CM—harvested from 5 day confluent irradiated HFF cultures) with 10 ng/mL FGF2 and mixed with a collagen I hydrogel. For the HES3, and hIPS lines 10 μM Y-27632 was also added to the medium. The collagen I matrix was formulated with acid-soluabilized bovine collagen I (Devro) with an equi-volume of 2×DMEM (Gibco) and neutralized using 0.1 M sodium hydroxide. The hPSC/collagen I matrix was formulated to give a final collagen I concentration of 1 mg/ml and 5×10⁵ hPSC per 170 μl. For the HES3 and hIPS lines 1×10⁶ and 0.5×10⁶ cells were used respectively per 170 μl. For each BHM, 170 μl of the hPSC/collagen I matrix was pipette into circular molds (i.d.=4 mm, o.d.=10 mm) fabricated using poly(dimethylsiloxane) (Sylgard, Dow Corning). After 10 min of culture in the incubator at 37° C. the collagen gelled and 1.25 ml of 1:1 human foreskin fibroblast-conditioned medium with 10 ng/ml FGF2 was added per BHM. The following day the BHMs were rinsed with RPMI medium and then differentiated as indicated in each figure, with 1.25 ml of medium per BHM. At day 13 the BHM were transferred onto mechanical stimulators as indicated. The protocol details for each figure are outlined in FIG. 8.

Cell Disassociation

2D cultures were disassociated by rinsing with PBS followed by incubation for 1 h in 1 mg/ml collagenase type I (Sigma) with 20% fetal bovine serum (FBS, Applied Biosystems) in PBS. The cells were then collected in a tube, rinsed with PBS and incubated with 0.25% Trypsin-EDTA (Applied Biosystems) for 5 min followed by rinsing with FBS containing medium.

For the initial BHM digestion protocol, BHMs were disassociated in 0.025 mg/ml to Liberase™ (Roche), 30 mM 2,3-butanedione monoxime at 37° C. for 60 min in PBS. To preserve cell surface markers, BHM was disassociated using the same protocol as for the 2D digestion.

Quantitative PCR (qPCR)

Cells, BHMs or human heart biopsies were harvested and stored at −80° C. until RNA extraction using Trizol following manufacturer's instructions (Applied Biosystems). 1 μg of RNA was then treated with DNAse (Roche) followed by cDNA synthesis using a High Capacity cDNA Reverse Transcription Kit (Applied Biosystems). qPCR was performed using Fast SYBR Green Master Mix (Applied Biosystems) on a 384-well format AB7900 HT (Applied Biosystems). Gene expression was normalized using 2^(−ΔCt) or 2^(−ΔΔCt) using GAPDH as the housekeeping gene which we found to be consistently expressed between conditions in all of our experiments. Primer details are given in below Table 1.

TABLE 1 Gene Size (Acc #) F R Purpose (bp) GAPDH CCTCAAGATCATCA- ATGTTCTGGAGAGCCCCGC qPCR 189 (NM_002046.3) GCAATGCC (SEQ ID (SEQ ID NO: 2) RT-PCR NO: 1) OCT4 CAGTGCCCGAAAC- GGAGACCCAGCAGCCTCA- qPCR 161 (NM_002701, CCACAC (SEQ ID NO: 3) AA (SEQ ID NO: 4) NM_203289, NM_001173531) NANOG CAGAAGGCCTCA- ATTGTT- qPCR 111 (NM_024865) GCACCTAC (SEQ ID CCAGGTCTGGTTGC (SEQ NO: 5) ID NO: 6) REX1 CACCGCCTCCCTTGG- TGTTCTGTTCA- qPCR  83 (NM_174900.3) GAATTCAG (SEQ ID CACAGGCTCCAGC (SEQ ID NO: 7) NO: 8) DNMT3B GGCCCAAGTAAAC- ATGCCTGGTGTCTCCCTT- qPCR 168 (NM_175848.1 CTAGCTCGGC (SEQ ID CATGC (SEQ IOD NO: 10) NM_175849.1 NO: 9) NM_006892.3) MIXL1 CCGAGTCCAG- CTCTGACGCCGAGACTTGG qPCR  58 (NM_031944) GATCCAGGTA (SEQ ID (SEQ ID NO: 12) NO: 11) NKX2-5 ACAACTTCGT- GTGGACACTCCCGAGTT- qPCR  82 (NM_001166175.1, GAACTTCGGCG (SEQ GCTCT (SEQ ID NO: 14) NM_001166176.1, ID NO: 13) NM_004387.3) TBX5 TCATAACCAAGGCTG- GCCCGTCACAGAC- qPCR 152 (NM_000192.3, GAAGG (SEQ ID NO: CATTTAT (SEQ ID NO: 16) NM_080717.2, 15) NM_181486.1, NM_080718.1) ISL1 CGCCTTGCAGAGTGAC GGACTGGCTACCAT- qPCR 147 (NM_002202.2) ATAG (SEQ ID NO: 17) GCTGTT (SEQ ID NO: 18) MYH6 CTCCTCCTACGCAACTG CGACACCGTCTGGAAG- qPCR  85 (α-MHC) CCG (SEQ ID NO: 19) GATGA (SEQ ID NO: 20) (NM_002471) MYH7 GACCAGATGAAT- GGTGAGGTCGTT- qPCR  63 (β-MHC) GAGCACCG (SEQ ID GACAGAACG (SEQ ID NO: (NM_000257) NO: 21) 22) MLC2v GGCGCCAACTCCAACG ACGTTCA- qPCR 149 (NM_000432.3) TGTT (SEQ ID NO: 23) CTCGCCCAAGGGC (SEQ ID NO: 24) ACTA1 (Skeletal actin ACCCAGATCATGTTT- TCATAAATGGGCACGTT- qPCR 143 (NM_001100.3) GAGACC (SEQ ID NO: GTG (SEQ ID NO: 26) 25) NPPA (ANP) TCTGCCCTCCTAAAAA TGTCCTCCCTGGCTGTTAT qPCR 156 (NM_006172.3) GCAA (SEQ ID NO: 27) C (SEQ ID NO: 28) CASQ2 TCTTGCAGGGCAGAAG GGACCTGGGCCACAAGCTC qPCR 205 (NM_001232.3) AGGGG (SEQ ID NO: AA (SEQ ID NO: 30) 29) NEUROD1 AGCCAC- GCGTGCCTCTAATCATGAA qPCR 143 (NM_002500.3) GGATCAATCTTCTC A (SEQ ID NO: 32) (SEQ ID NO: 31) CDH1 (ECAD) GAACGCATT- ATTCGGGCTTGTT- qPCR 118 (NM_004360.3) GCCACATACAC (SEQ GTCATTC (SEQ ID NO: 34) ID NO: 33) CDH2 (NCAD) CCTGGAACGCAGTG- TGGTTTGACCACGGTGAC- qPCR 104 (NM_001792.3) TACAGA (SEQ ID NO: TA (SEQ ID NO: 36) 35) SNAIL1 AGCGAGCTGCAG- GGACAGAG- qPCR 136 (NM_005985.3) GACTCTAA (SEQ ID TCCCAGATGAGC (SEQ ID NO: 37) NO: 38) MESP1¹ AGCCCAAGT- AAGGAACCACTT- qPCR  82 (NM_018670.3) GACAAGGGACAACT CGAAGGTGCTGA (SEQ ID (SEQ ID NO: 39) NO: 40) SOX17¹ AGGAAATCCTCA- CCCAAACTGTTCA- qPCR 111 (NM_022454.3) GACTCCTGGGTT (SEQ AGTGGCAGACA (SEQ ID ID NO: 41) NO: 42) ATP2A2 (SERCA2) ACCTCATCTCGTCCAA TGTCACCAGATTGACCCAG qPCR 110 (NM_001681.3 CGTC (SEQ ID NO: 43) A (SEQ ID NO: 44) NM_170665.3) PLN ACAGCTGCCAAGGCTA TCCATGATACCAGCAGGAC qPCR 114 (NM_002667.3) CCTA (SEQ ID NO: 45) A (SEQ ID NO: 46) RYR2 TGCAAGACTCACCGAA CCACCCAGACATTAGCAGG qPCR 125 (NM_001035.2) GATG (SEQ ID NO: 47) T (SEQ ID NO: 48) COL1A1 GTGCTAAAGGTGCCAA ACCAGGTTCACCGCTGTTA qPCR 128 (NM_000088.3) TGGT (SEQ ID NO: 49) C (SEQ ID NO: 50) COL3A1 CCAGGAGCTAACGGTC CAGGGTTTCCATCTCTTCC qPCR 103 (NM_000090.3) TCAG (SEQ ID NO: 51) A (SEQ ID NO: 52) COL5A1 GACACCTCCAACTCCT AGTGAACTCCCCCTCCAAG qPCR  72 (NM_000093.3) CCAA (SEQ ID NO: 53) T (SEQ ID NO: 54) COL4A1 GTTGGTCTACCGGGAC GTTCACCTCTGATCCCCTG qPCR 145 (NM_001845.4) TCAA (SEQ ID NO: 55) A (SEQ ID NO: 56) LAMC1 GTGAGAGGTGCCGAGA GTGCTTAGAGAGCCCACAG qPCR  88 (NM_002293.3) GAAC (SEQ ID NO: 57) G (SEQ ID NO: 58) TGFB2 CGAACCCAAAGGG- TAAGCTCAGGAC- qPCR  91 (NM_001135599.2, TACAATG (SEQ ID NO: CCTGCTGT (SEQ ID NO: NM_003238.3) 59) 60) TTN Ex49-50 (ALL GTAAAAAGAGCTGCCC GCTAGGTGGCCCAGTGCTA qPCR  68 TTN)² CAGTGA (SEQ ID NO: CT (SEQ ID NO: 62) (NM_001267550.1 61) NM_001256850.1 NM_133437.3 NM_133432.3 NM_003319.4) TTN Ex107-108 CAGCAGAACTCAGAAT ATCAAAGGACACTTCACAC qPCR 110 (N2BA)² CGA (SEQ ID NO: 63) TC (SEQ ID NO: 64) (NM_001267550.1 NM_001256850.1 NM_133378.4) TTN Ex50-219 (N2B)² CCAATGAGTATGGCAG TACGTTCCGGAAGTAATTT qPCR  93 (NM_133437.3 TGTCA (SEQ ID NO: GC (SEQ ID NO: 66) NM_133432.3 65) NM_003319.4) endoOCT4³ CCTCACTTCA- CAGGTTTTCTTT- RT-PCR 164 (NM_002701, CTGCACTGTA (SEQ ID CCCTAGCT (SEQ ID NO: NM_203289, NO: 67) 68) NM_001173531) endoSOX2³ CCCAGCAGACTTCA- CCTCCCATTT- RT-PCR 151 (NM_003106.3) CATGT (SEQ ID NO: CCCTCGTTTT (SEQ ID NO: 69) 70) endoKLF4³ GATGAACTGACCA- GTGGGTCATA- RT-PCR 145 (NM_004235.4) GGCACTA (SEQ ID NO: TCCACTGTCT (SEQ ID NO: 71) 72) endoMYC³ TGCCTCAAATTG- GATTGAAATTCTGTG- RT-PCR 192 (NM_002467.4) GACTTTGG (SEQ ID TAACTGC (SEQ ID NO: 74) NO: 73) SeV GGATCACTAGGTGA- ACCAGACAAGAGTT- RT-PCR 181 TATCGAGC TAAGAGATATGTATC (SEQ ID NO: 75) (SEQ ID NO: 76) BS OCT4-2⁴ TTAGGAAAATGGG- TAC- BS 296 TTAGTAGGGATTT (SEQ CCAAAAAACAAATAAAT- ID NO: 77) TATAAAACCT (SEQ ID NO: 78) BS OCT4-4⁴ GGATGTTATTAAGAT- CCTAAACTCCCCTTCA- BS 406 GAAGATAGTTGG (SEQ AAATCTATT (SEQ ID NO: ID NO: 79) 80) ¹Kattman, et al. Cell Stem Cell 8, 228-240 (2011). ²Neagoe, et al. Circulation 106, 1333-1341 (2002). ³Park, et al. Nature 451, 141-146 (2008). ⁴Freberg, et al. Mol Biol Cell 18, 1543-1553 (2007).

Immunostaining

Digested cardiac differentiation cells were plated on 0.1% gelatin coated glass coverslips for 24 h in 20% FBS (Gibco) in RPMI 1640 supplemented with 1 mmol/L sodium pyruvate, 100 IU/ml Penicillin and 100 μg/ml Streptomycin. The cells were then fixed in Histofix (Roti) for 10 min at room temperature. The cells were then blocked for 30 min in 5% FBS, 1% bovine serum albumin (Sigma) and 0.5% Triton X-100 (Sigma) in PBS (blocking buffer). The cells were then stained with primary antibodies in blocking buffer for 90 min followed by secondary antibodies in blocking buffer and Hoechst for 60 min at room temperature (Table 2). Stained cells were imaged using a Zeiss 710 confocal microscope.

TABLE 2 Antibodies and stains Antibody/Stain Supplier Cat No. Marker Dilution IgG₁ RnD Systems MAB002 Control 1:100 FC 1:100 IF TRA-1-60-FITC BD Pharmingen 560173 Pluripotency 1:100 FC (mouse) 1:100 IF OCT3/4 (rabbit) Santa Cruz Bio- sc-9081 Pluripotency 1:200 FC technology 1:200 IF NANOG (goat) RnD Systems AF1997 Pluripotency 1:20 IF α-smooth muscle Sigma A2547 Stromal cell (used 1:4000 FC actin (mouse) predominately in 1:400 IF this study) α-smooth muscle Abcam ab32575 Stromal cell (used 1:50 FC actin (rabbit) only with α- actinin co-staining) Collagen I Abcam ab34710 Stromal cell 1:2000 FC (rabbit) 1:500 IF NKX2-5 (rabbit) Santa Cruz Bio- H-114 Cardiomyocyte/ 1:200 IF technology cardiac progenitor α-actinin (mouse) Sigma A7811 Cardiomyocyte 1:4000 FC 1:1000 IF Cardiac Troponin I Millipore MAB1691 Cardiomyocyte 1:200 IF (mouse) SIRPA (mouse) Biolegend 323802 Cardiomyocyte 1:100 FC PDGFRα (mouse) RnD Systems MAB1264 Mesodermal progenitor 1:25 FC CD90 (mouse) RnD Systems MAB2067 Stromal cell 1:125 IF 1:500 FC Donkey anti-goat Invitrogen A-11056 NA 1:400 IF Alexafluor 546 Goat anti-mouse Invitrogen A-11001 NA 1:1000 FC Alexafluor 488 1:400 IF Goat anti-rabbit Invitrogen A-11010 NA 1:1000 FC Alexafluor 546 1:400 IF Phalloidin Invitrogen A22283 NA 1:50 IF Alexafluor 546 Hoechst33342 Invitrogen H3570 NA 1:1000 FC 1:1000 IF (FC—Flow cytometry, IF—Immunofluorescence)

Whole-Mount Immuno-Staining

BHMs were fixed in Histofix for 2-4 h at 4° C. The BHMs were then stained with primary antibodies for 2-3 days followed by secondary antibodies and Phalloidin 546/Hoechst for 2-3 days at 4° C. (Table 2). Stained BHMs were imaged using a Zeiss 710 confocal microscope.

Flow Cytometry

Cells were stained either live or fixed using Histofix for 10 min at room temperature or ethanol. The cells were stained in 5% FBS in PBS (membrane blocking buffer) for cell surface markers (excluding TRA-1-60) and blocking buffer for internal markers. The cells were then stained with primary antibodies in blocking buffer for 45 min followed by secondary antibodies in blocking buffer and Hoechst for 30 min at 4° C. (Table 2). A BD LSRII was used for flow cytometry analysis (BD Biosystems). Live cells populations were gated based on forward-side scatter profiles; fixed cells populations were gated based on Hoechst staining. BD FACSDiva Software (BD Bioscience) or Cyflologic v1.2.1 (Cyflo Ltd) were used for analysis.

Contraction Measurements

Contraction experiments were performed in organ baths at 37° C. and continuous bubbling with 5% CO₂/95% O₂ to maintain a physiological pH in Tyrode's solution containing (all in mM): 120 NaCl, 1 MgCl₂, 0.2 CaCl₂, 5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucose and 0.56 ascorbate. Calcium was adjusted using a 0.2 M calcium chloride solution. All BHM were first analysed at 3 Hz with 5 ms square pulses of 200 mA in order to pace at approximately the embryonic heart rate. BHM were mechanically stretched at intervals of 125 μm until Lmax, i.e. the tissue length were maximum twitch tension/force of contraction was recorded in the presence of maximally inotropically active calcium concentrations (2 mmol/L; Frank-Starling mechanism). Subsequently, BHM were subjected to different calcium concentrations (0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mM) and the twitch force recorded. For isoprenaline experiments the calcium concentration was adjusted to 0.6 mM and subsequently the isoprenaline concentration was adjusted to 1 μM.

Oxygen Diffusion Profile

The oxygen diffusion profile was generated using numerical analysis of a pseudosteady-state approximation of cylinder diffusion with oxygen consumption dependence on concentration (Eq. 1). Parameters from the literature (Brown et al. Biotechnol Bioeng 97, 962-075 (2007)) and as determined in previous experiments were used (Table 3). Numerical analysis and plotting was performed using MATLAB V12 (Mathworks) using the solver bvp4c and the Singular Term option.

$\begin{matrix} {0 = {{\frac{- D_{O\; 2}}{r}\left\lbrack {r\frac{{dC}_{O\; 2}}{dr}} \right\rbrack} + {V_{\max}\rho_{cardiomyocyte}e^{(\frac{- \alpha}{C_{O\; 2}})}}}} & {{Eq}.\mspace{14mu} 1} \end{matrix}$

C_(O2)— oxygen concentration as a function of radial position, r—radial position in cylinder, D_(O2)—oxygen diffusion constant, V_(max)—maximal oxygen generation rate by cardiomyocytes, ρ_(cardiomyocyte)—density of cardiomyocytes, α—constant for oxygen generation rate dependence on oxygen concentration.

TABLE 3 Oxygen diffusion model parameters Model Parameter Value D_(O2) 2 × 10⁵ cm²/s P_(cardiomyocyte) $\rho_{cardiomyocyte} = \frac{{CellNumber} \cdot {CardiomycoytePurity}}{BHMVolume}$ V_(max) −5.44 × 10⁻⁸ nmol/cell/s α Calculated to fit oxygen consumption-concentration relationship from ⁵; 1.14 μM Bounding Conditions ${{{At}\mspace{14mu} r} = 0};{\frac{{dC}_{O\; 2}}{dr} = 0}$ At r = R; C_(O2) = C* R = 0.5 mm; C* = 61 μM

5. Brown, et al. Biotechnol Bioeng 97, 962-975 (2007).

Statistical Analysis

All data is presented as mean±s.e.m. Appropriate statistical analyses were used for each data set as indicated in the Figure legends using Graph Pad Prism or Microsoft Excel.

Custom-Made Supplement to Replace B27®

TABLE 4 Custom-made supplement (CMS) to replace B27. Final Substance concentration 25x Supplier Albumin 5 mg/ml 125 mg/ml Sigma, A9511 Transferrin 10 μg/ml 250 μg/ml Sigma, T0665 EthanolamineHCl 2 μg/ml 50 μg/ml Sigma, E6133 Sodium selenite 0.032 μg/ml 0.8 μg/ml Sigma, S5361 L-CarnitineHCl 4 μg/ml 100 μg/ml Sigma, C0283 Hydrocortisone 1 μg/ml 25 μg/ml Sigma, H2270 Fatty acid supplement 0.5 μl/ml 12.5 μg/ml Sigma, F7050 Triiodo-L-thyronine 0.004 μg/ml 0.1 μg/ml Sigma, T 6397 Prepare 25x in cell culture-qualified water.

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1.-15. (canceled)
 16. A bioengineered heart muscle (BHM) produced by a method comprising the steps of (i) cultivating pluripotent stem cells in a basal medium comprising an effective amount of (a) BMP4, Activin A, FGF2, a GSK3-inhibitor, and (b) a serum-free supplement resulting in a final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, and 0.0001-0.1 μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm differentiation of said pluripotent stem cells; (ii) cultivating the cells obtained in step (i) in a basal medium comprising an effective amount of an inhibitor of the Wnt-signaling pathway and a serum-free supplement as in (i), thereby inducing cardiac differentiation of the cells; and (iii) cultivating the cells obtained in step (ii) in a basal medium comprising an effective amount of a serum-free supplement as in (i), under mechanical stimulation, thereby promoting cardiac maturation.
 17. The BHM of claim 16, wherein the BHM a) is capable of being paced at multiple frequencies up to at least 3 Hz; or b) exhibits an increased twitch tension in response to increased resting length and resting tension; or c) exhibits a calcium EC₅₀ higher than 0.2 mM; or d) exhibits a twitch tension of more than 200 μN; or e) exhibits an inotropic response to 1 μM isoprenaline of more than 40 μN under paced conditions at 0.6 mM calcium; or f) comprises cardiomyocytes and CD90⁺ stromal cells; or g) has a combination of at least two of (a) to (f).
 18. The BHM of claim 16, wherein the pluripotent stem cells are selected from embryonic stem cells, induced pluripotent stem cells, and parthenogenetic stem cells.
 19. The BHM of claim 16, wherein the pluripotent stem cells are pluripotent stem cells of primate origin.
 20. The BHM of claim 16, wherein the pluripotent stem cells are human pluripotent stem cells.
 21. The BHM of claim 16, wherein the basal medium in step (i) comprises 1-20 ng/ml BMP4; 0.1-10 ng/ml FGF2; and 1-20 ng/ml Activin A.
 22. The BHM of claim 21, wherein the basal medium in step (i) comprises about 5 ng/ml BMP4; or about 5 ng/ml FGF2; or about 9 ng/ml Activin A; or a combination thereof.
 23. The BHM of claim 16, wherein the GSK3-inhibitor in the basal medium of step (i) is selected from the group consisting of CHIR99021, CHIR98014, SB216763, TWS119, Tideglusib, SB415286, and LY2090314.
 24. The BHM of claim 23, wherein the basal medium in step (i) comprises 0.1-10 μM CHIR99021.
 25. The BHM of claim 16, wherein the inhibitor of the Wnt-signaling pathway in the basal medium of step (ii) is selected from the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR-3, IWR-4, IWR-5, XAV939, DKK1, quercetin, ICG-001, pyrvinium, CCT031374, iCRT-3, 5, 14, CPG049090, NC043.
 26. The BHM of claim 25, wherein the basal medium of step (ii) comprises 0.1-10 μM IWP4.
 27. The BHM of claim 16, wherein the basal medium of step (iii) a) further comprises 0.1-10 ng/ml TGFβ1; or b) does not comprise an effective amount of FGF2; or c) comprises 0.5-3 mM Ca²⁺; or d) comprises a combination of any one of a), b), and c).
 28. The BHM of claim 27, wherein the basal medium of step (iii) comprises about 1 ng/ml TGFβ1; or about 1.2 mM Ca²⁺; or a combination thereof.
 29. The BHM of claim 16, wherein the mechanical stimulation in step (iii) is dynamic mechanical stimulation or static stretch.
 30. The BHM of claim 29, wherein the mechanical stimulation in step (iii) is dynamic mechanical stimulation.
 31. The BHM of claim 16, wherein the basal medium of any one of step (i), (ii) or (iii) comprises 10-1000 μM of ascorbic acid or a salt or a derivative thereof.
 32. The BHM of claim 16, wherein the basal medium used in any one of step (i), (ii) or (iii) is DMEM/F12, Iscove's medium, αMEM, DMEM, and RPMI.
 33. The BHM of claim 32, wherein the basal medium used in any one of step (i), (ii) or (iii) is RPMI supplemented with pyruvate.
 34. The BHM of claim 16, wherein the method comprises prior to step (i) a seeding step, wherein said pluripotent stem cells are seeded in a ratio of (2.5-6×10⁶ cells/1 mg collagen)/1 ml medium in a suitable mould.
 35. The BHM of claim 34, wherein the seeding step is carried out 18-30 h prior to step (i).
 36. The BHM of claim 34, wherein the medium used in the seeding step further comprises a ROCK-inhibitor.
 37. The BHM of claim 36, wherein the ROCK-inhibitor is selected from Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil, GSK429286A, and RKI-1447.
 38. The BHM of claim 36, wherein the medium used in the seeding step comprises 1-50 μM Y27632.
 39. The BHM of claim 16, wherein said pluripotent stem cells are not produced using a process which involves modifying the germ line genetic identity of human beings or which involves use of a human embryo for industrial or commercial purposes. 